Evidence has been obtained suggesting that P. shermanii contains acetyl CoA carboxylase. Therefore, it appears likely that malonyl CoA needed for fatty acid synthesis in these bacteria is formed by the carboxylation of acetyl CoA rather than the previously considered mechanism involving its synthesis by the reaction mediated by oxaloacetate transcarboxylase. The carboxylase levels were highest during logarithmic growth of P. shermanii. As the cells entered stationary phase, the carboxylase levels dropped rapidly reaching undetectable levels within 48 hours of this phase. The carboxylase reaction was shown to require Mn-ATP, HCO minus over 3 and acetyl CoA. Besides carboxylating acetyl CoA, the P. shermanii enzyme could also carboxylate propionyl CoA and butyryl CoA but not hexanoyl CoA. The reaction proceeded equally well when ATP was replaced by dATP. However, other purine and pyrimidine triphosphates (GTP, UTP, ITP) and PPi were essentially inactive.